1. Field of the Invention
The present invention relates generally to a liquid crystal display device, and more particularly to a dot-matrix-type liquid crystal display device which can perform color display.
2. Description of the Related Art
A liquid crystal display device has various features such as thickness in size, light weight, and low power consumption. The liquid crystal display device is applied to various uses, e.g. OA equipment, information terminals, timepieces, and TVs. In particular, a liquid crystal display device comprising thin-film transistors (TFTs) has high responsivity and, therefore, it is used as a monitor of a mobile TV, a computer, etc., which displays a great deal of information.
In recent years, with an increase in quantity of information, there has been a strong demand for higher image definition and higher display speed. Of these, the higher image definition is realized, for example, by making finer the array structure of the TFTs.
On the other hand, in order to increase the display speed, consideration has been given to, in place of conventional display modes, an OCB (Optically Compensated Bend) mode, a VAN (Vertically Aligned Nematic) mode, a HAN (Hybrid Aligned Nematic) mode and a π alignment mode, which use nematic liquid crystals, and an SSFLC (Surface-Stabilized Ferroelectric Liquid Crystal) mode and an AFLC (Anti-Ferroelectric Liquid Crystal) mode, which use smectic liquid crystals.
Of these display modes, the VAN mode, in particular, has a higher response speed than in the conventional TN (Twisted Nematic) mode. An additional feature of the VAN mode is that a rubbing process, which may lead to a defect such as an electrostatic breakage, can be made needless by vertical alignment. Particular attention is drawn to a multi-domain VAN mode (hereinafter referred to as “MVA mode”) in which a viewing angle can be increased relatively easily.
In the MVA mode, for example, mask rubbing and pixel electrode structures are devised, or a protrusion is provided within a pixel. Thereby, the inclination of an electric field, which is applied to the pixel region from the pixel electrode and counter-electrode, is controlled. The pixel region of the liquid crystal layer is divided into, e.g. four domains such that the alignment directions of liquid crystal molecules are inclined at 90° to each other in a voltage-on state. This realizes improvement in symmetry of viewing angle characteristics and suppression of an inversion phenomenon.
In addition, a negative retardation plate is used to compensate the viewing angle dependency of the phase difference of the liquid crystal layer in the state in which the liquid crystal molecules are oriented substantially vertical to the major surface of the substrate, that is, in the state of black display. Thereby, the contrast (CR) that depends on the viewing angle is improved. Besides, more excellent viewing angle/contrast characteristics can be realized in the case where the negative retardation plate is a biaxial retardation plate having such an in-plane phase difference as to compensate the viewing angle dependency of the polarizer plate, too.
In the conventional MVA mode, however, the compensation of the viewing angle at the time of displaying intermediate gradations or white is insufficient, and the gradation characteristics are different between a case in which the liquid crystal display device is observed in its normal direction and a case in which the liquid crystal display device is observed in an oblique direction to the normal direction. Consequently, in a case where the liquid crystal display device, which performs multi-color display, is observed in an oblique direction, intermediate gradations are degraded and become generally whitish. For example, in a MVA mode liquid crystal display device in which one pixel is divided into four domains, the luminance at the time when intermediate gradations are displayed and observed in an oblique direction would become substantially equal to the luminance at the time of white display, and the intermediate gradations are visually recognized as generally whitish display.
To cope with this problem, there has been proposed a technique in which one frame, which displays an image, is divided into a plurality of sub-frames, and the gradation levels are controlled on the basis of the luminances in the respective sub-frames (see, e.g. N. kimura et al., “60.2: Invited Paper: New Technologies for Large-Sized High-Quality LCD TV”, SID'05 DIGEST, pp. 1734-1737 (2005)). In addition, there has been proposed a technique in which each pixel is divided into a plurality of sub-pixels, and the gradation levels are controlled on the basis of the luminances in the respective sub-pixels (see, e.g. Sang Soo Kim, “66.1: Invited Paper: The World's Largest (82-in.) TFT-LCD”, SID'05 DIGEST, pp. 1842-1847 (2005)).